Formation of Massive Population III Galaxies through Photoionization Feedback: A Possible Explanation for CR7

Abstract

We explore the formation of massive high-redshift Population III (Pop III) galaxies through photoionization feedback. We consider dark matter halos formed from progenitors that have undergone no star formation as a result of early reionization and photoevaporation caused by a nearby galaxy. Once such a halo reaches $\approx 10^9{M}_{\odot }$, corresponding to the Jeans mass of the photoheated intergalactic medium (IGM) at $z\approx 7$, pristine gas is able to collapse into the halo, potentially producing a massive Pop III starburst. We suggest that this scenario may explain the recent observation of strong He II 1640~Å~line emission in CR7, which is consistent with $\sim 10^7{M}_{\odot }$ of young Pop III stars. Such a large mass of Pop III stars is unlikely without the photoionization feedback scenario, because star formation is expected to inject metals into halos above the atomic cooling threshold ($\sim 10^8{M}_{\odot }$ at $z\approx 7$). We use merger trees to analytically estimate the abundance of observable Pop III galaxies formed through this channel, and find a number density of $\approx 10^{-7}\mathrm{Mp}{\mathrm{c}}^{-3}$ at $z=6.6$ (the redshift of CR7). This is approximately a factor of ten lower than the density of Ly$\alpha$ emitters as bright as CR7.

5 pages, 4 figures, replaced with version accepted by MNRAS Letters

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• Population III star formation in a Lambda CDM universe, II: Effects of a photodissociating background
• Identification of the brightest Lyα emitters at z=6.6: implications for the evolution of the luminosity function in the re-ionisation era
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• The critical radiation intensity for direct collapse black hole formation: dependence on the radiation spectral shape
• Minihalo photoevaporation during cosmic reionization: evaporation times and photon consumption rates
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